1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, an image processing program, and a storage medium storing an image processing program with which it is possible to deal with feature information that is unique to individual sheets of printed documents (hereinafter referred to as paper fingerprints).
2. Description of the Related Art
Recently, the quality of printed copies has improved due to color printing functions and enhanced resolutions of copying machines, so that it is often difficult to distinguish an original from a copy by the naked eye. Thus, techniques for distinguishing an original from a copy on the basis of features of fiber patterns of individual sheets have been proposed. This serves to prove the authenticity or inauthenticity of a sheet of document, assure originality, or facilitate management.
In this case, a sheet of document that is to be managed as an original is scanned by a scanner, and a slight shade pattern of fibers of the sheet is registered as a feature specific to the sheet, or the pattern is written to the sheet itself depending on cases. The feature specific to the sheet is considered as information unique to each sheet, similar to a human fingerprint. This feature or information derived from the feature will be referred to herein as a paper fingerprint.
In a conventional system, when information has already been printed on a sheet of document, a blank area where information is not printed on the sheet of document is detected, and a fiber pattern in the area of the sheet is read to extract a fiber pattern feature.
In order to extract a feature of a fiber pattern unique to each sheet, a blank area of a certain size is desired. For example, when the resolution of a scanner is 600 dpi, assuming a sheet of an ordinary material, usually, a blank area of 512 pixels×512 pixels, i.e., 22×22 mm, may be used as a paper fingerprint area. As the size of the paper fingerprint area for extracting a feature increases, it is more likely that information unique to each sheet can be extracted. This reduces the possibility of incorrectly recognizing different sheets as the same sheet. That is, the probability of obtaining the same result as a previous result increases when a feature is extracted again.
Furthermore, if a paper fingerprint is extracted from only one area, the result of subsequent extraction of a paper fingerprint could differ from the result of the first extraction due to, for example, dirt, scar, or intentional writing that may be present on the sheet after obtaining a paper fingerprint. Thus, it is desired to define a plurality of paper fingerprint areas on one sheet to deal with such cases. By defining a plurality of paper fingerprint obtaining areas on one sheet, the probability of proper conservation and maintenance of at least one of the paper fingerprint obtaining areas is improved. A value indicating the probability of proper conservation and maintenance will be referred to as a paper fingerprint intensity.
As described earlier, as the number of areas on the sheet where paper fingerprints can be obtained increases, the paper fingerprint intensity for assuring originality increases. That is, as the number of areas where paper fingerprints can be obtained decreases, the paper fingerprint intensity for assuring originality decreases, so that the sheet is not suitable for an important document that requires originality assurance.
However, in a conventional system, when a user causes a scanner or the like to read a paper fingerprint for originality assurance, the degree of paper fingerprint intensity of the relevant sheet of document is not recognized. Thus, if the intensity of a paper fingerprint obtained is low, it is likely that recognition of the original fails when the originality is checked later. Furthermore, in an extreme case, when a background or a copy-forgery-inhibited pattern is printed over the entire surface of a sheet so that it is not possible to obtain a paper fingerprint in any area, recognition of the original could fail when the originality is checked later. As a result, it is possible that the user is able to register a sheet of document in a conventional originality assurance system even though it may not be possible to obtain a paper fingerprint with a sufficient intensity from the sheet of document, so that recognition of the original could fail when the originality is checked later.